Method of reconditioning used sand



- rammed May 1950 asoaas'i J iusrnon F REcoNmT'IoNmG USED sl m) Marshall V. Chamberlin, Saginaw, Micln, mm...

to The Dow Chemical Company, Mich, a. corporation oi Delaware Application February 15, 1947, Serial No. 728,967 I,

4 Claims. (01. 22-217) Nollrawing.

The invention relates to casting light metal, such as magnesium and magnesium-base alloys and aluminum and aluminum-base alloys. It more particularly concerns an improved sand casting method for use with light metals.

' In the conventional practice 01' sand casting light metals, the molds are either of green sand or baked sand and may include one or morebaked cores. After theinetal is cast, the moldsare destroyed, and, in :the, case of the green sand molds, the sand is ordinarily reused. However, it hasnot been found feasible with existing methods to reuse the sand of baked molds. This is because the used sand-o1 baked molds is contam-* I mated withthe residuum ofthebirlder-uSGd in the original molding sand mixture and its presence spoils the permeability and other molding properties of the sand. In order to render such used sand suitable for reuse, the spent binder must be removed and this can be accomplished only by a high temperature calcination or leachingor both. The use of dry or baked sand molds for casting light metals, although highly desirable from the standpoint of making high qualit castings, has not been widely used owing to the necessity (with conventional methods) of rejecting the used sand after each casting or applying costly and laborious methods 01 reclaiming the sand.

The principal object of the invention is to provide a method of casting light metals in which the necessity for either rejecting the used sand or applying costly and laborious reclaiming methods on the used sand are avoided.

Other objects and advantages will appear as the description of the invention proceeds.

My invention is predicated upon the discovery that by forming a molding sand mixture of foundry sand, a reversible organic colloid, such as,a' cereal flour asa binder, and water for tempering, molds made therefrom can be baked to form baked sand molds and then, after-the molds have been used for casting light metal, they may be crushed and the crushed molding sand mixture rendered lit for reuse merely by retempering with water and adding, if desirable, a small amount of additional binder and sand to make up for losses. Thus, the sand mixture maybe used repeatedly and a succession of high grade castings of light metal may be made with great economy of labor and material.

The invention, then, consists of the method hereinafter fully described and particularly pointed out in the claims.

In carrying out the invention, ordinary foundry sand may be used having a suitable permeability,

ing strength to the mold.

Midland,

the weight of the sand. A binder 01' a reversible organic colloid, such as a cereal flour, is added.

Various cereals or mixtures of them may be used. in flour form. Examples of such binders are com flour, oat flour, wh'eat'flour, rye flour, and the like. These materials'either disperse or dissolve in the water added to the sand to form the binding agent holding the sand grains together during molding and harden when baked, .thus strongly cementing the sand grains together thereby giv- All such reversible organic colloids derived from cerealsh'ave the property of forming solutions or dispersions in water, and, after beingsubjected to a, drying and baking operation whereby they become hardened, are again capable of being dispersed or dissolved in water. In addition, these materials are some what sticky when wet and thus render the sand moldable and when dried adjacent to the sand grains, as in baking the molded sand, cement the sand grains together. These materials are, therefore, capable of being repeatedly alternately rendered sticky by tempering with water and hardened by baking. The proportion of binder used affects the strength of the molded mixture and its permeability to gas and is so chosen as to give the results desired as regards these factors. Generally, the amount to use is in the range of 0.1 to 2 per cent of the weight of the sand. I have found also that a combination of two or more cereal binders are more satisfactory than one cereal. Dextrin, or other equivalent gummy material derived from cereals, when-used with the cereal binder, I have found also improves the baked strength of the molded mixture. The term reversible is thus used herein to mean the property possessed by the binder of being resoluble or redispersible in water after the water has been baked out of the solution or dispersion.

In addition to the water and binder, it is desirable to employ in the molding sand mixture a relatively small amount 01 a water-soluble polyhydric alcohol, such as ethylene glycol, glycerine, and the like, as such materials improve the moldability of the sand mixture. A suitable amount is between about 0.1 and 1 per cent of the weight of the sand.

For casting magnesium and its alloys, conventional inhibitorsv of the oxidation of magnesium hours.

. "3 l as in conventional practice. Examples 01 these agents are: ammonium salts, e. g. NH4C1; fluorides, e. g. NHQFJIF; silicofluorides, e'. g.

(NHO 2815's I 4 4'. The following example is fllustratlve oi asuitable composition for an initial batch of molding sand for use according to the invention:

. borofluorides, e. g. NHiBlfi; sulfur; and boric acid.

Suitable proportions depend upon the agent used the sand weight, although other proportions are sometimes used.

The sand, binder, water, and, the other addimixed. The mixture is formed into moldsasin the usual foundry operations with lightmetal.

and may be between about 0.1 and 4 per cent of l tives', it used, such as oxidation inhibitors and j polyhydric alcohol, are intimately and uniformly If desired, the cores,,it any are used in the mold, Y

may be molded of the same sand mixture.

The molds, and cores it any are made of the sand mixture, are then an jected to a .baking operation which sets the bin er and thus hardens and strengthens them. 'Baking is accomplished by heating, preferably in an oven, at a suitable drying temperature above about 212 F.- but belowthe decomposition temperature'of. the binder, until the binder hardens- Toavoid 'carbonization or decomposition of the-binder and loss of oxidation inhibitor if present,the baking tern perature should not exceed about 350 F. The time required to harden the molds depends upon their size and may be a few minutes to several Castings are made inv the bakedor hardened molds in the usual way, and, after the castings sand is reclaimed and made ready for reuse.

Reclaiming is accomplishedby sifting the used sand, if necessary so as to remove chills, if any,

corewire, etc., and then the bond of baked binder betweenthe sand grains is fractured so that the individual grains are freed and a ,substantially free-flowing sand is obtained. The bond between the sand grains may be broken by mulling the sand or otherwise subjecting the sand to a pulverizing treatment as by the use of a ball mill which separates the sandgrains from...

.each other but does not substantially fracture the grains themselves. The reclaimed used sand 1 is made ready for reuse by retempering with water and other tempering liquid, e. g. polyhydric alcohol, if desired, and incorporating with the sand additional binder, oxidation inhibitor,

)andsome new sand to replenish losses in so far as necessary. l

The amount of additional materials needed to replace losses varies according to the nature of v the castings produced and is more or less proportional to the ratio of the. weight of the casting to that of the sand used for its mold. In general, about 5 to per cent of the binder and about 10 to 40 per cent of the oxidation inhibitor are destroyed during casting and nearly all the tempering liquid is vaporized and lost during baking? Accordingly, appropriate restoration is made when reworking the sand so that an adequate concentration of these materials is present in the sand at the time of molding and casting. In addition, there is always some loss of the sand itself which is carried away with the castings, mold boxes, cores, etc., and as fines and dust in amount up to about 5 to about per cent .of the weight 01' the sand used ineach mold and appropriate replacement is made as already indicated.

In some instances, higher sand losses may occur as with very complicated castings and those with extremely heavy sections.

, are shaken out of the molds, the used molding.

The foregoingingredients are mulled together until thoroughly intermingled, whereby a moldable sand mixture is obtained. This operation usually takes from about 4 to 10minutes. Table 1 sets forth pertinent data as regards certain properties of the sand mixture of the above example when formed into a mold and also after baking the mold for 30 minutes at"350 F.

TABLE I Propertiesoi molded new sandmimture As Molded As Baked Permeability l '80 110 Compression Strength p. s. L- l. 2 900 Hardness 92 Determined by the American Foundrymens Association Standard Method for Determining Permeability.

2 Dial reading of a standard Dietert core hardness tester.

As already indicated, some of the sand mixture is lost with. each casting sothat' as the sand mixture is recycled, through the operation of molding casting and reclaiming, a certain amount .of new sand is added- (together with other additives as already explained) at each recycling. As illustrative of this, a succession of test castings were made consisting of a magnesiurn'cylinder about 3 inches in diameter and 12 inches long andweighing about 12.5. pounds, the mold for which required about 85 pounds of the sand mixture having an initial composition as indicated in the example. In these tests, it was found that for each remolding of the sand,

I new sand was required at the rate of about 25 pounds of new sand for each pounds of used or reclaimed sand;' and for each pounds of the mixture of used and new sand about 0.2 part of boric acid, 0.3 part of sulfur, 0.12 part of cereal flour, 0.24 part of dextrin, 0.5 part of ethylene glycol, and 4.0 parts of water were required to, restore the molding qualities. The restored moldm properties thus obtained are set forth in- Table II.

I I Team I! Properties of molded reclaimed sand mixture As Molded As Baked Permeability i. no Comgresslon. p. s. i. 1. 15 950 Hm ess 90 Determined by the American Foundrymens Association Standard Method for Determining Permeability. Dial reading of a standard Dietert core hardness tester.

Referring toTable II, it is apparent that by the addition of a comparatively small amount of new binder, and inhibitor, and the proper amount of tempering liquid, themolding quality of the sand .ingthecastings 8 is readily restored so that it is substantially the same as, if not better than, that of new sand.

Among the advantages of the invention are that (1) a given batch of sand may be used repeatedly and its volume maintained by the addition of a small proportion of new sand after each molding, thus conserving the use of sand; (2) relaally smooth surface castings are obtained which are relatively free from the usual casting defects; (7) in the case of magnesium castings, all the advantages of baked mold casting are obtained together with the added advantage of good protection from oxidation; (8) the problem of the disposal of large quantities of used molding sand is avoided.

I claim:

1. The method of making a succession of castings of a light metal in baked sand molds which comprises mulling together used sand derived from a later step in the method, said used sand containing as used a binder of a heat-hardenable, reversible organic colloid selected from the group consisting of com flour, oat flour, wheat flour, rye flour, and dextrin and being substantially free from other binders, fresh clean sand to replace losses occurring during the practice of the method, a sufficient amount of a binder of a heathardenable, reversible organic colloid selected from the group consisting of corn flour, oat flour, wheat flour, rye flour, and dextrin to render the said fresh sand moldable, and a tempering liquid in amount sufllcient to render the resulting mulled mixture moldable; forming molds of the mulled mixture: baking the molds so as to harden the binder, thereby cementing the sand grains together and vaporizing the tempering liquid; casting light metal inthe baked molds; separati'rmn the baked molds; and returningtheusedsandofthe bakedmoldstothe aforesaid mulling step.

6 2. The method according to claim 1 in which a combination of at least two heat-hardenable reversible organic colloidal binders selected from the group consisting of corn flour, oat flour, wheat flour, rye flour, and dextrin is used.

3. The method according to claim 2 in which the combination of at least two heat-hardenable Legrsible organic colloidal binders includes dex- 4. The method of making a succession of castings of magnesium and alloys thereof which comprises mulling together used sand derived from a ,later step in the method said used sand containing as used a binder of a cereal flour and dextrin and being substantially free from other binders; fresh clean sand to replace losses occurring during the practice of the method, the amount of said fresh clean sand not exceeding about 35 per cent of the weight of the used sand; about 0.1 to

1.0 per cent of a binder of cereal -ilo'ur and dextrin based upon the weight of the fresh clean sand; about 0.1 to 1.0 per cent of a polyhydric alcohol, about 2 to 7 per cent of water, and from about 0.1 to 2 per cent of an inhibitor of the oxidation of magnesium, the proportions of the water, alcohol, and inhibitor being based upon the total weight of the sand mixture; forming molds of the mixture; baking the molds so as to harden the binder and vaporize the water and alcohol; casting magnesium or alloy thereof in the so baked molds; separating the castings from the molds and returning the-used sand 01' the molds to the aforesaid mulling step.

MARSHALL V. CHAMBERLIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,533,892 Osborne Apr. 14, 1925 1,602,412 Runyan Oct. 12, 1926 1,700,713 Campion Jan. 29, 1929 2,881,242 Wood Aug. 7, 1945 2,422,118 Meyer June 10, 1947 2,444,413 Weston July 6, 1948 OTHER REFERENCES The Foundry, May 1943, pages 96, 97 and 183- 85. 

1. THE METHOD OF MAKING A SUCCESSION OF CASTINGS OF A LIGHT METAL IN BAKED SAND MOLDS WHICH COMPRISES MULLING TOGETHER USED SAND DERIVED FROM A LATER STEP IN THE METHOD, SAID USED SAND CONTAINING AS USED A BINDER OF A HEAT-HARDENABLE, REVERSIBLE ORGANIC COLLOID SELECTED FROM THE GROUP CONSISTING OF CORN FLOUR, OAT FLOUR, WHEAT FLOUR, RYE FLOUR, AND DEXTRIN AND BEING SUBSTANTIALLY FREE FROM OTHER BINDERS, FRESH CLEAN SAND TO REPLACE LOSSES OCCURRING DURING THE PRACTICE OF THE METHOD, A SUFFICIENT AMOUNT OF A BINDER OF A HEATHARDENABLE, REVERSIBLE ORGANIC COLLOID SELECTED FROM THE GROUP CONSISTING OF CORN FLOUR, OAT FLOUR, WHEAT FLOUR, RYE FLOUR, AND DEXTRIN TO RENDER THE SAID FRESH SAND MOLDABLE, AND A TEMPERING LIQUID IN AMOUNT SUFFICIENT TO RENDER THE RESULTING MULLED MIXTURE MOLDABLE; FORMING MOLDS OF THE MULLED MIXTURE; BAKING THE MOLDS SO AS TO HARDEN THE BINDER, THEREBY CEMENTING THE SAND GRAINS TOGETHER AND VAPORIZING THE TEMPERING LIQUID; CASTING LIGHT METAL IN THE BAKED MOLDS; SEPARATING THE CASTINGS FROM THE BAKED MOLDS; AND RETURNING THE USED SAND OF THE BAKED MOLDS TO THE AFORESAID MULLING STEP. 